The present invention relates generally to a method for reducing the level of an undesired signal, generated through frequency converter devices, by implementing a local oscillator (hereinafter "LO") leak cancellation circuit. More particularly, the present invention relates to a novel feedback correction loop, for a radio frequency (RF) transmitter or receiver, to reduce an undesired local oscillator leak generated through an RF frequency converter device, such as a mixer or modulator.
There is an accelerating demand for highly integrated radio frequency transmitter and receiver designs, lower cost, increased accuracy, greater dynamic range, etc. A radio frequency transmitter modulates the information to be communicated onto a carrier, amplifies the signal to the desired power level and delivers it to the transmitting antenna. A radio-frequency oscillator's signal is modulated by the message signal so as to shift the frequency from a baseband (BB) or intermediate frequency (IF) to the desired RF frequency prior to transmission. RF transmitters offer a unique combination of high output power, low distortion and programmable gain to satisfy the requirements of the upstream communications link in broadband access terminals.
However, RF transmitters typically suffer from unwanted signal transmission. As noted above, a local oscillator (LO) is used in conjunction with a frequency converter device, i.e., a frequency modulator or mixer, to translate the signal spectrum to an alternate band (BB or IF to RF). One common undesired signal generated in a RF transmitter, the Local Oscillator leak (LO leak) signal, is generated through the RF frequency converter devices. Typically, it is not necessary to reduce the level of these undesired signals, as the rejection of the carrier signal of 30-40 dB for example, is usually acceptable for most applications. However, in an application that requires a better than 30-40 dB carrier rejection, a method of adjusting the DC balance in the mixer or modulator, so as to correct for the LO leak and improve the carrier rejection, must be devised.
The present invention is therefore directed to the problem of developing a LO leak cancellation circuit and a method for reducing the level of undesired signal transmission.